I. Field of the Invention
This invention relates generally to means for obtaining active media of gas lasers, and more particularly to discharge tubes useful for a certain class of gas laser referred to as "rare-gas-metal-vapor laser" which utilizes an excited level of singly ionized metal atoms as the upper laser level.
II. Description of Prior Art
Forms of discharge so far proposed for the active media of rare-gas-metal-vapor lasers are classified into two classes. The first class is referred to as "positive column" discharge utilizing a long positive column part of a dc glow discharge in a capillary as an active medium in which a weak axial field is sustained to keep up a necessary and sufficient electronic temperature for compensating the diffusion-determined charge loss by various ion formation processes. The positive column discharge is very stable and keeps an uniform metal ion distribution throughout the positive column by virtue of so called "cataphoresis effect" when metal vapor is introduced at the low potential end of the positive column, as shown by J. P. Goldsborough in a paper appeared in Applied Physics Letters vol. 16, pp. 159-161 (1969).
The second class is referred to as "hollow cathode" discharge utilizing a negative glow part of a dc discharge. On account of the fact that a negative glow usually is not likely to be developed into a long column, in contrast to the positive column, various structures are proposed as an efficient laser active medium.
A "coaxial hollow cathode" laser described by W. K. Schuebel in a paper appeared in IEEE Journal of Quantum Electronics vol. QE-6, pp. 574-575 (1969) has a long hollow cathode with slots in its wall and tubular anode arranged coaxially to the hollow cathode. In this case the negative glow parts near the slots develop into a long column inside the hollow cathode in a high current region as it is know as "hollow cathode effect".
A "multiple anode hollow cathode" laser described by Y. Sugawara and Y. Tokiwa in a paper appeared in Japanese Journal of Applied Physics vol. 9, pp. 588-589 (1970) and that by J. A. Piper and C. E. Webb in a paper appeared in Journal of Physics D: Applied Physics vol. 6, pp. 400-407 (1973) each has a long hollow cathode with many holes in its wall and has corresponding numbers of anodes. An array of negative glows in the hollow cathode near each hole serves as an active medium for the laser.
The hollow cathode discharge are known to work efficiently as an active medium at higher metal vapor pressure region of up to several tenths of a torr, because there is unperishable resource of high energy electrons in a negative glow supplied from a sheath surronding the cathode. This resource is independent of metal vapor pressure, in contrast with the positive column where there is a remakable lowering of the electronic temperature at a relatively low metal vapor pressure influenced by the lowering of the axial field conditioned by increasing supply of ions by introducing metal vapor.
In accordance with the fact that the hollow cathode discharge is effective at a higher metal vapor pressure region, production of a high output power, production of spectra with much restricted excitation processes, and hard-to-be-saturable nature of a laser active media are found to be realized by rare-gas-metal-vapor laser using a hollow cathode discharge.
Regardless of such an attractive feature of the hollow cathode discharge as an active media for the rare-gas-metal-vapor lasers, it has two fatal disadvantages.
The first fatal disadvantage is that there is no efficient method to guarantee the uniform distribution of metal vapor throughout the long hollow cathode. The second fatal disadvantage is the unstable nature of the long hollow cathode discharge. A part of coaxial hollow cathode is likely to turn into an arc discharge spot in its high current operation. Whereas, currents of different anode in a "multiple anode hollow cathode" discharge is not likely to balance with each other even with high stabilizing resistance connected to each anode.
The reason why no commercially available hollow cathode laser is so far produced is quite understandable in connection with these fatal disadvantages.